1. Field of the Invention
This invention relates to motor control apparatus and, more particularly, to a circuit which is adapted to sense a motor overload condition whereupon motor operation is controlled to alleviate that condition, such as by reversing the motor, reducing the drive current supplied to the motor or interrupting the drive current so as to stop the motor.
2. Description of the Prior Art
Motor control circuitry has long been used in driving, regulating and controlling the operation of an electric motor. Often, such motor control circuitry is intended to provide protection against motor damage and, for those applications in which the motor is used with user-operated devices, to prevent personal injury to the user.
One example of a controllable motor adapted for use in a human environment is found in recording/reproducing apparatus, such as a video tape recorder (VTR). Many commercially available VTR's employ a cassette holder to receive a tape cassette which then is loaded into the VTR. Typically, an electric motor is mechanically coupled to the cassette holder and is driven to move that cassette holder between a load/unload position whereat the tape cassette may be loaded onto or unloaded from the holder, and a record/playback position whereat the tape cassette is brought into operable relationship with electromechanical record/reproducing apparatus. Usually, the simple actuation of a START switch energizes the motor to drive the cassette holder, either from its load/unload position to its record/playback position or from its record/playback position to its load/unload position. It is recognized, however, that uncontrolled motor operation may lead to personal injury of the user. For example, the user may accidentally locate his finger in the path of the cassette holder as it moves from its load/unload position to its record/playback position, resulting in possibly severe damage or painful injury.
To minimize this risk of personal injury, various safeguards have been proposed to prevent the user from inadvertently placing his finger in the path of motor-driven apparatus. Unfortunately, even the most detailed precautions often are defeated or, at the least, result in complicated and often difficult manipulation by the user in loading or unloading a tape cassette from its holder. Accordingly, as an alternative, protection arrangements have been proposed which stop a cassette loading operation should the user's finger become entangled in the equipment. While this may prevent further injury, such arresting apparatus often relies upon mechanical elements which, while stopping the movement of the drive motor nevertheless does nothing about the continued supply of drive current thereto. As a result, the motor may be subjected to a current overload condition in which excess motor current flows therethrough. Consequently, the motor may be damaged and may burn out.
Another difficulty associated with mechanical protection devices is attributed to the fact that such devices often are quite large and bulky and are not easily incorporated into portable VTR's, such as video cameras. In particular, 8 mm video cameras which record on and play back from an 8 mm cassette are quite small and cannot easily accommodate the aforementioned protection devices.
In recognition of the danger of motor overload, some relatively simple techniques have been proposed to prevent the motor from burning out due to excess motor current drawn thereby. For example, the motor may be designed to exhibit limited torque which also limits the current; or the motor drive circuit may impose an inherent limitation on the maximum current that may be supplied thereby. Such proposals limit the power that can be exerted by the motor and often require a motor and motor drive circuit that are relatively expensive to design and manufacture.
One simple alternative to special motor design is the use of a fuse connected in series between the power supply from which the motor is driven and the motor drive circuit, or even the motor itself. While a fuse may prevent the motor from drawing excess current and, thus, avoid burn-out, a "blown" fuse must be replaced, and this may result in frequent servicing of the motor-driven equipment. In the environment of consumer equipment, it may be difficult for a user to carry out even the relatively simple task of replacing a fuse, thus making it more difficult for the user himself to maintain and service his equipment.
An alternative to the use of a simple fuse relies upon a variable power supply which is controlled to reduce the current supplied thereby to the motor when an overload condition is sensed. For example, when overload is sensed, the power supply voltage may be reduced to prevent motor burn-out. However, if a common power supply is used to provide energy both for the motor and for other circuits used with the motor-driven equipment, a change in the power supply voltage which is intended to prevent motor burn-out may induce malfunctions or faults in other circuitry that is supplied with that same power supply voltage. To avoid this possibility, a separate power supply is needed for the exclusive use of the motor and motor drive circuitry. This, however, is expensive, adds to the complexity of the overall equipment, and increases the size and weight of the motor-driven apparatus.
One proposed protection circuit which is intended to prevent motor burn-out yet does not require a separate power supply and does not vary the power supply voltage that is used by other circuitry connects a current limiting circuit in series between the motor drive circuit and the power source from which motor current is derived. The current limiting circuit includes active devices, such as FET's, bipolar transistors, or the like, and functions to limit the current flowing from the power supply to the motor. The current limiting circuit is sensitive to current flowing therethrough to the motor such that when this motor current increases in magnitude, the impedance of the current limiting circuit likewise increases. Hence, the current permitted to flow through this circuit to the motor is limited so as not to exceed some predetermined level.
Unfortunately, the use of a current limiting circuit of the aforementioned type increases the power dissipation of the overall circuitry due to the fact that the impedance connected in series with the power supply increases. Furthermore, the use of transistors and other active elements in the current limiting circuit inherently dissipates power even when such devices are quiescent, as when the motor current is well below the aforementioned predetermined level. Consequently, if the motor and motor protection circuitry are used in portable equipment, the battery pack power source normally used with that equipment will exhibit a shorter effective life because of the increased power dissipation in the current limiting circuit. Moreover, such current limiting circuitry is rather bulky and, thus, is unsuitable for portable, compact equipment such as portable video cameras.